Digesters are pressure cooking vessels used to process cellulosic fibrous material, such as wood chips. The digester applies pressure and cooking liquor to process the chips into cellulose pulp from which paper products may be made. A dual-phase or vapor-phase digester includes a pressure vessel that is partially filled with a slurry of the cellulosic fiber and cooking liquor and has a super-atmospheric steam zone (also referred to as the gas zone) above the slurry. Since this gas zone is compressible compared to the liquid zone below it, the pressure within a vapor-phase digester is typically determined by the pressure of the gas at the top of the digester.
Prior art vapor-phase digesters are illustrated in U.S. Pat. Nos. 6,024,837; 3,380,883; 3,429,773; 3,532,594; 3,578,554 and 3,802,956. Recently-issued U.S. Pat. No. 6,024,837 is commonly owned with this application, and is incorporated by reference in its entirety.
In a vapor-phase digester, the wood chips are typically heated by exposure to steam as the chips are introduced to the steam-filled zone at the top of the digester. A vapor-phase digester directly exposes chips to steam for heating by having a chip level that is above the level of the liquid in the digester. The chips are exposed to steam as they are distributed in the vessel by a top separator device and as the rest on the top of the chip pile.
Vapor phase digesters are known to have inverted top separators. Top separators are used to convey a slurry of wood chips, or other cellulosic material, into a pressure vessel of a continuous digester. The chips are “cooked” as they move from the top of the chip pile and progress down into the slurry through the digester vessel, which is generally a vertical column oftentimes having a height greater than 100 feet (33 meters). The inverted top separator is mounted in the top section of the digester. The top section of the vessel contains a vapor portion, and the portion of the vessel below the vapor is filled with a liquid-chip slurry.
Wood chips, or other comminuted cellulosic fibrous material, are typically fed to the inlet of a continuous digester using a separate feed system. The chip feed system typically includes equipment for de-aerating, heating, and pressurizing the chips, and introducing cooking liquor to the chips to form a slurry of chips and liquor that flow to the top separator of the digester. In a vapor-phase digester, the chip slurry is introduced into a top separator which distributes the chips to a gas space at the top of the digester. An inverted top separator removes some cooking liquor from the slurry before distributing the wood chips outward at the top of the vessel. The top separator distributes the chips into the steam of the upper section of the digester vessel. The wood chips overflow the top separator and fall, through the steam, to the chip pile below the separator.
A top separator is an inverted screw conveyor. The slurry of chips and liquor are fed to the bottom of the conveyor, via a conduit that is connected to the chip feed system external to the digester vessel. The chips are carried upward in the screw conveyor. As the chips overflow the top of the conveyor, they fall freely in the steam-filled atmosphere of the upper section of the vapor-phase digester. A screen drum surrounds the screw conveyor to allow excess liquid to flow out of the chips being conveyed to the top of the separator.